This invention relates to plug-in sensor units for monitoring systems and more particularly, to a supervisory arrangement for such sensor units.
In monitoring systems for large building complexes, it is common practice to provide an alarm control panel at a central location for supervision of each of a plurality of sensor units located throughout the building complex and connected to the alarm control panel by two or more conductors, typically a pair of conductors for extending electrical power to the sensor units and one or more additional conductors for supervisory purposes.
Various sensor mounting arrangements are available for such sensor units. Direct-wire sensor units are designed for mounting directly on a mounting surface with hard-wired connections between the sensor unit and the alarm control panel. In a common arrangement plug-in sensor units are used. This can include a two-piece assembly including a detector base which is hard-wired into the monitoring system and a plug-in detector head which is constructed and arranged for snapping into the detector base. The use of plug-in sensor units not only facilitates installation and maintenance, but also permits sensor units to be interchanged as necessary affording numerous advantages and increasing the flexibility of the system. The detector bases may be installed in new construction without exposing the detector sensing heads to plaster, dust and other contamination in buildings nearing completion. Plug-in units are particularly suitable for use in installations that have many detector heads, and high ceiling installations and environments where detectors may require frequent maintenance or cleaning. The sensor units can be serviced, tested and cleaned more easily than hard-wired surface-mounted units, especially when mounted on high ceilings, because the detector head can be removed without a ladder using suitable tools. Also, if a plug-in detector head is faulty or unsuitable for a given location, it can simply be unplugged and readily replaced by a compatible unit.
A shortcoming of the plug-in sensor unit is its susceptibility to tampering. Although anti-tampering arrangements are sometimes employed, if a person attempting to defeat the system should be successful in removing a detector head, the absence of the detector head may not be noticed for a long period of time. Also, removal of the detector head may affect the operation of the monitoring system.
Accordingly, arrangements have been proposed for the supervision of detector head installations. In one known arrangement, a shorting contact is provided on each detector base. The shorting contact is manually operable to interconnect a pair of terminals of the detector base, thereby connecting the detector base in series with the power conductors and with other sensor units in the system. This shorting contact is operated to open this circuit path which is then completed by a conductive shorting loop on the detector head, by a mechanical cam on the detector head when the detector head is plugged into the detector base. However, the detector head does not operate the shorting contact to its connection position when a detector head is mounted on or removed from the base. Thus, if the detector head is subsequently removed, the shorting contact is not operated, interrupts the power circuit for the sensors of the system, and the voltage to detectors further down the zone is interrupted. This condition is indicated on the alarm control panel.
The shorting contact may be closed manually during system installation or service when the detector head is not in place. Electrical supervision of detector installation at that location is then disabled.
In this prior art arrangement, mounting of the detector head on the detector base causes disruption of the shorting loop by operating the spring contact out of engagement with a terminal on the detector base. The shorting contact is spring loaded and the cam on the bottom of the detector head releases the spring contact to its open position when the detector head is installed on the detector base. Also, because the electrical circuit path is completed through a further shorting loop on the detector head, a separate connection is required between the detector head and the detector base to provide this supervisory arrangement. Because the electrical connection is provided within the detector head, this both complicates the wiring connection arrangement and requires a separate plug/connector for plug-in sensor units of this type.